An explicit method for the analysis of transient compressible flow in pipe networks / Frederick Botha

Abstract:

This thesis deals with the development of an explicit pipe network analysis
code for the benchmarking of Flownet. Flownet is a general-purpose thermal-fluid
network analysis code used as a tool in the design of the Pebble Bed Modular Reactor.
The National Nuclear Regulator (NNR) requires the use of benchmarks to verify and
validate such a design code. RELAPS was selected as an additional commercial code
to benchmark Flownet.
Various benchmark cases were identified to determine the validity of specific
models and components. Some of the models or components validated include pipes,
boundary conditions, heat transfer, non-inertial elements and junctions. Comparisons
were also done to validate the integrated use of these components in an arbitrary
structured pipe network. For virtually all steady-state flow cases, Flownet compared
very well with analytical results. Only for isothermal flow at high Mach numbers
were some differences noted. Some anomalies were identified with the use of
RELAPS.
Comparisons between the three codes for transient flow were very favourable,
except for cases were the anomalies iniRELAP5 came into play. For these cases
Flownet and Lax-Wendroff still compared very well. The only noteworthy anomaly
for transient flow was found when solving total temperature at pipe boundaries. This
anomaly was only found at boundaries with a combination of little or no flow and
sharp pressure transients. A difference in boundary modelling between Flownet and
Lax-Wendroff is most likely the cause of this difference. This localized effect had
virtually no impact on the solution elsewhere in the network.
Upon successful completion of the selected benchmark cases, one can
conclude that Flownet uses a valid implementation of the basic components tested.
Further, these models are also integrated in a way that enables valid modelling of
complex pipe networks.